This invention relates to the continuous coating of sheet or strip articles, such as aluminum or other metal strip, hereinafter generically termed xe2x80x9cstrip articles.xe2x80x9d In an important sense, it is directed to coating heads especially suitable for applying low solvent or reduced solvent coatings or solvent free coatings, e.g. molten polymers, and to coating lines and methods employing such heads.
More particularly, the present invention is directed to improvements in the types of strip-coating apparatus and methods described in U.S. Pat. No. 4,675,230 and in International (PCT) Application No. PCT/CA94/00291 published Dec. 8, 1994, under International Publication No. WO 94/27739.
U.S. Pat. No. 4,675,230 describes apparatus and procedure for applying a paint or like coating to an elongated strip article using a coating head with a die that has a slit to which coating material is supplied under pressure and a strip-facing extended surface or metering land immediately downstream of the slit, and a support such as a roll around which the strip is advanced past the head for receiving from the slit a layer of paint metered between the die and the strip, wherein a load is continuously exerted on the die during operation for urging the land against the applied paint layer on the strip so as to maintain a uniform metering gap between the land and the coated strip surface. The load may be exerted by devices such as air cylinders acting on the die and capable of adjustment to vary the magnitude of the load for different coating operations. In this way, a coating of superior uniformity can readily be applied to a surface of an article such as sheet metal strip, notwithstanding that the strip characteristically exhibits some variation in thickness along its length; the maintenance of a load on the coating head die facilitates accuracy and ease of setup, and also enables the die to conform positionally to variations in strip thickness, for maintaining a constant metering orifice aperture. U.S. Pat. No. 5,147,462 describes apparatus for automatic film thickness control in coating procedures and apparatus of the same general type.
The methods and apparatus of U.S. Pat. No. 4,675,230 are shown as arranged for applying a coating to one major surface of a strip article (one-sided coating). The aforementioned PCT publication describes modifications or arrangements of methods and apparatus of this type enabling simultaneous application of coatings to both major surfaces of a moving strip or sheet (two-sided coating). The apparatus arrangements thus described for two-sided coating comprise two coating heads of the general type described in U.S. Pat. No. 4,675,320, mounted face-to-face with the strip passing between them.
In particular, the aforementioned PCT publication discloses apparatus for continuous two-sided coating of a strip article, including means for defining a path of continuous longitudinal advance of an elongated strip article having opposed major surfaces to be coated, the path including a rectilinear portion in which the opposed surfaces of the strip article are substantially planar; two coating heads each including a die defining an elongated open-sided slit and having a land portion with an extended land surface immediately adjacent the exit side of the slit, for respectively depositing layers. of liquid coating material on the opposed major surfaces of a strip article advancing in the path; and means for supplying liquid coating material under pressure to the slit of each head. The apparatus thus disclosed also includes means for supporting the dies of the two heads in facing relation to each other on opposite sides of the rectilinear portion of the strip article path while permitting individual translational movement of at least the land portion of at least one of the dies relative to the supporting means in a direction perpendicular to the major surfaces of a strip article advancing in the path. This supporting means positions the heads such that their die slits respectively open toward the opposed major surfaces of an advancing strip article in the rectilinear path portion with their long dimensions extending transversely of the path; their die land surfaces are disposed, substantially in register with each other, beyond the slits in the direction of strip article advance, respectively facing the opposed major surfaces of an advancing strip article and converging toward each other and toward the path in the direction of article advance; and, during operation with the article advancing past the slits and liquid coating material supplied to both slits as aforesaid, the article major surfaces respectively drag layers of coating material from the slits, the layers being thereby deposited on the article major surfaces. In combination with the foregoing features, the disclosed apparatus further includes means for continuously exerting a load on at least the land portion of the aforementioned one die during operation as aforesaid such that the layers of coating material deposited on the strip major surfaces are pressed between the land surfaces of the dies of the heads to maintain the deposited coating layers at predetermined constant thicknesses while the coating layers alone hold the dies entirely away from contact with the strip article major surfaces.
In some embodiments of the described apparatus, the aforementioned one die comprises a land portion as defined above and a portion that is stationary during operation, the land portion being movable relative to the stationary portion and cooperating therewith to define the slit. The load-exerting means, in these embodiments, acts between the supporting means and the movable land portion of the die. In other embodiments, the aforementioned one die is formed integrally and the load-exerting means acts between the supporting means and the entire die, which is supported on a bearing so as to be capable of moving as a unit relative to the supporting means during operation. The other die (including its land portion) can be held entirely stationary during the coating operation, or, alternatively, the dies of both heads (or their land portions) can be movable relative to the supporting means during operation and can both be acted on by load-exerting means.
In the heads of the aforementioned U.S. Pat. No. 4,675,230 and PCT publication, the load-exerting means for the (or each) die conveniently comprises at least one air cylinder acting thereon. Preferably, the load-exerting means comprises a plurality of air cylinders acting at points spaced along the length of the die, and the die is made sufficiently flexible to conform to variations of strip thickness across the width of the strip, for enhanced coating uniformity.
Heretofore, coating heads for use in the apparatus and methods described in the aforementioned patents and publication have been designed with a relatively small cross-section so that the complete die (including land portion) of the head could be flexed by modest applied forces. To achieve the required degree of flexibility for conforming to strip thickness variations as just described, the overall cross-sectional dimensions of the die should not exceed about four inches (10 cm) and should preferably be less than two inches (5 cm). Slot dies for solvent-borne coatings can readily be designed to meet this requirement. However, for solvent free coatings (solvent free molten polymer coatings), the dies must incorporate heating elements and must have specially designed, polished channels to ensure uniform distribution of the coating. For these reasons, typical polymer extrusion dies have overall cross-sectional dimensions in excess of six inches (15 cm). It would not be possible to control the flexing of a die with these dimensions, without resorting to very high loading forces.
The present invention, in a first aspect, broadly contemplates the provision, in apparatus for continuously coating a major surface of a strip article while the article is advancing longitudinally along a defined path, of a coating head including a die defining an elongated slit, with an entry side and an exit side, for depositing a layer of liquid coating material on a facing major surface of a strip article advancing along the path, and having a land portion with an extended land surface at the slit exit side; means for supplying liquid coating material to the slit; means for supporting the die with the slit and land surface facing the path and the slit extending transversely of the path such that during coating operation a major surface of a strip article advancing in the path past the slit drags a layer of coating material from the slit, the supporting means permitting movement of the die relative thereto in a direction normal to the path and to the slit; and means for continuously exerting a load on at least the land portion of the die during operation as aforesaid such that the layer of coating material deposited on the last-mentioned article major surface is pressed between the land surface and the last-mentioned article major surface to maintain the layer at a predetermined constant thickness while the layer alone holds the die entirely away from contact with the last-mentioned article major surface; the die including a main body defining at least the entry side of the slit, and the land portion being so connected to the main die body as to be capable of flexing relative thereto in the aforesaid normal direction but to resist flexing in the direction of article advance in the path.
It will be understood that, when the die is supported as described above relative to an advancing strip article, the land portion (at the exit side of the slit) extends transversely of the strip article major surface to be coated. In this coating head, the defined connection of the land portion to the main body of the coating head die affords the characteristic flexing properties of the metering portion of a coating head of the type described in U.S. Pat. No. 4,675,230, even though the main body of the die may have the increased bulk and thickness needed to accommodate built-in heating elements for solvent free molten polymer coatings, and may therefore be incapable of flexing. At the same time, the movability of the entire die in the aforesaid normal direction relative to the supporting means enables coarse positional adjustment of the die, so that the tolerances of the head and support components do not have to be as exact as would be necessary if only the land portion were movable.
As further particular features of the invention, the land portion is advantageously formed integrally with the main body of the die, and the load-exerting means exerts the load directly on the land portion. The land portion can be cantilevered with respect to the main die body, or, in currently preferred embodiments of the invention, the land portion can be flexibly connected to the main die body on two opposed sides of the land portion respectively toward and away from the slit (upstream and downstream of the land surface, as stated with reference to the direction of strip advance in the path).
Moreover, in currently preferred embodiments of the coating head of the invention, the load-exerting means comprises a piston movable in the aforesaid normal direction and means connected to the piston for transmitting the load directly to the land portion, the last-mentioned means being interengageable with the main die body for permitting but limiting the extent of motion of the piston and land portion relative to the main body in the aforesaid normal direction. In particular, the land portion may have a second surface opposed to the land surface; the transmitting means may include a chock attached to the piston, a rod for load-transmitting engagement with the second surface of the land portion, and a lateral projection (such as a pin or dovetail protection) carried by the chock; and the main die body may include a socket (recess or aperture) within which the projection is received, the projection and socket being positioned and dimensioned to permit a limited extent of motion of the projection within the socket in the aforesaid normal direction. Also, in these embodiments, the transmitting means can transmit a force from the piston to the main die body for moving (lifting) the die relative to the supporting means in the normal direction.
The invention in a further aspect contemplates the provision of a coating line for continuously coating a major surface of a strip article while the article is advancing longitudinally along a defined path, comprising the combination of a head as described above with means for defining a path and direction of continuous longitudinal advance of a strip article having a major surface to be coated. The coating line. may be arranged to coat one or both major surfaces of a strip article; in a line for coating two opposed surfaces of a strip article, the path includes a rectilinear portion in which the opposed surfaces are substantially planar, the supporting means supports the above-described coating head at that rectilinear portion of said path such that during a coating operation the slit faces one of the opposed major surfaces of a strip article advancing along the path, and a second coating head is disposed at the rectilinear path portion in opposed relation to the first-mentioned coating head, for depositing a layer of liquid coating material on the other of the opposed major surfaces of the strip article.
In yet another aspect, the invention contemplates the provision of a method of continuously applying a layer of liquid coating material to a major surface of an elongated flexible strip article, utilizing a coating line incorporating the above-described coating head of the invention. This method, again, may be used to effect coating of one or both major surfaces of a strip article very advantageously, it can be used to coat one or both major surfaces of a strip article with a solvent free liquid coating material, such as a molten polymer.
Further features and advantages of the invention will be apparent from the detailed description hereinbelow set forth, together with the accompanying drawings.